The overall focus of this program is directed toward a better understanding of cellular regulation. The individual components of the program are: (1) bone marrow transplantation with reference to histocompatibility antigens not linked to HLA, autocytotoxic cells, and immunocompetence of transplant recipients; (2) suppressor cells; (3) human serum growth factor isolated from platelets; (4) regulation of RNA metabolism by growth factors and drugs; (5) drug-induced perturbations in purine and pyrimidine metabolism; and (6) differentiation in erythroid culture systems.